Variable magnification stereo optical system



35o-427 SR bLArUH KUUM March 24, 1970 n. M. MULLER 3,502,392

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INVENTOR. (pzfr /y/V/aff irme/ers l United States Patent O 3,502,392 VARIABLE MAGNlFICATION STEREO OPTICAL SYSTEM Robert M. Muller, Cheektowaga, N.Y., assigner to American Optical Corporation, Southbridge, Mass., a corporation of Delaware Filed Feb. 9, 1968, Ser. No. 704,369 Int. Cl. G02b 15/14 U.S. Cl. 350--184 ABSTRACT F THE DISCLOSURE A continuously variable magnification stereo optical system including two identical optical sub-systems, the axes of which intersect at an acute angle. Along each axis there is one fixed lens element and two lens elements which are movable relative to one another and to the fixed lens element. Each lens element consists of a cemented doublet and a singlet. The particular characteristics of al1 the singlets and doublets, including their radii, thicknesses, refractive indices and dispersions, result in a system having well corrected aberrations over the entire magnification range while still achieving the desired continuously variable magnification function.

'I'his invention relates to a continuously variable magnification optical system especially well suited for a stereomicroscope, and more particularly to such an optical system for the objective section of a stereomicroscope having an object positioned at a substantially xed distance from the image plane of the microscope.

In a stereomicroscope there is generally provided an optical system having identical optical su-b-systems with axes which are inclined to each other at an acute angle so that the image plane of each system is inclined with respect to the specimen stage. In one of the simpler types of such an optical system adapted for continuously variable magnification with fixed object and image planes, such as that disclosed in Schuma, Patent No. 3,057,259, issued Oct. 9, 1962, there are provided three cemented doublets along each optical axis. 'I'hese include a positive lens doublet that is closest to the object and which is held stationary, and another positive lens doublet and a negative lens doublet therebetween, the latter two doublets being moved axially with respect to each other and the stationary lens doublet. The movable positive and negative lens doublets are shifted along their respective axes at different rates in order that the magnification of the system be continuously variable and that the image plane be held fixed. Typically, a doubly-grooved barrel is associated with each axis and both barrels are rotated in common by rotation of a control knob. The two movable lens doublets on each axis are linked to cam followers which ride in the barrel grooves in order that the magnification be varied in accordance with the movement of the control knob. An improved mechanical structure, which is especially suitable for providing the proper mechanical functioning of the optical system of the present invention, is disclosed in the copending application of Olin W. Boughton, filed contemporaneously herewith and assigned to the assignee of this application.

Doublets as described above are used rather than singlets to correct to a suliicient extent the aberrations which would be introduced by single lens elements. Moreover, in the design of a stereo optical system, it is necessary to take into consideration the desired maximum numerical aperture, and the desired maximum field. Certain dimensional specifications of the system must also Ibe considered, such as the working distance and the object to image distance.

3 Claims ICC It is the major object 'of this invention to provide for a stereomicroscope, a continuously variable magnification objective optical system Awith fixed image and object planes in which -by the use of relatively few correcting lenses along each optical axis of the system, the desired correction for aberrations is achieved while obtaining a desired magnification range.

I have discovered that a highly corrected system can be achieved with the use of only three doublet-singlet pairs along each optical axis. While in general the use of three such paired lens elements along each optical axis is highly satisfactory, I have also discovered that near optimum performance can be achieved with the use of lenses having the specific characteristics disclosed in connection with the illustrative embodiment of the invention.

Other objects, features and advantages of my invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIG. 1 depicts schematically the optical system of my invention;

FIG. 2 is a table setting forth various characteristics of the lens elements illustrated in FIG. l; and

FIG. 3 is a graph illustrating the relative movement of the movable lens elements relative to the fixed lens element.

The optical system of FIG. l is shown only schematically since the mechanical aspects thereof do not form a part of the present invention. A mechanical structure particularly suitable for use with the optical system of the present invention is that disclosed in the above-identified Boughton patent application.

Referring now in detail to FIG. 1, an opticgLarrangement is shown having an opticsystem 8 for the objective section of a stereomicroscope. In an actually constructed instrument embodying the invention of the present application, means are provided for letically adjusting the optical system 8 with reference to a specimen stage so that the object plane lies in the desired plane of view of the specimen. However, such an arrangement forms no part of the present invention and such means are well known inthe art. Further, the terms stationary or fixed as hereinafter used'mean stationary and fixed subsequent to initial vertical adjustment of the optical system. The specimen stage of the system is designated by the reference numeral 10. The system 8 includes two linear optical axes 11, 12 with object planes 13, 15 respectively. The axes diverge from the object plane 10 at an acute angle of 10, the two axes intersecting at a point P on the specimen stage. The object planes 13, 15 intersect at this point. Because the optical axes 11 and 12 vare inclined to each other at said acute angle of 10, thus yielding a desired degree of stereoscopy, the respective object planes for each of the optical axes are inclined to the specimen stage at an angle of 5 thereto. The optical system is arranged so that the specimen stage 10 is located at an essentially constant distance from the image planes 34a and 34b of each axis.

Along each axis there are disposed three lens elements L1, L1 and L3. Each lens element consists of a cemented doublet and a singlet.l The lens elements along the optical axis 11 find identical counterparts in the lens elements along the optical axis 12, and the lens elements along each optical axis comprise identical optical sub-systems. Accordingly, only the lens elements along the optical axis 11 will be described in detail.

Lens element L1 is stationary and fixed with respect to the object plane 13. Lens elements L2 and L3 are movable along the axis relative to one another and to the fixed lens element L1. The two lens elements L2, L3 are shifted along the axis 11v by an associated barrel 16 having two circumferential camming grooves 17, 18 formed therein. Specifically, barrel 16 is axially mounted on shaft 15 which is driven by a bevel gear assembly comprising gears 31, 32. Gear 31 is on axis 30 as is the bevel gear 31A of the other optical sub-system. Both of these gears are mounted on a common shaft (not shown) which is in turn driven by a control knob (not shown).

Lens element L2 is mechanically linked to a cam follower 20, the linkage being indicated symbolically by the dotted line 22. Cam follower rides in the cam groove 18 in barrel 16 and as barrel 16 is rotated in either direction by the control knob and the bevel gears, the lens element L2 is shifted along the optical axis 11. Similarly, lens element L3 is linked, as shown at 21, to a cam follower 19 which rides in the cam groove 17 on barrel 16 and moves in the same manner as lens element L2. Lenses with the same reference numerals also move along optical axis 12 as a result of the cooperation of elements 15A through 22A, 31A and 32A. The displacements of lens elements L2, L3 at different rates and as a function of the angular rotation of barrels 16 and 16A are, of course, controlled by the grooves 17, 18 and 17A, 18A.

The magnification of the lens element L1 is essentially constant. The magnification of the lens element L3 varies to a relatively small extent. Most of the variation in the magnification of the system occurs from the variation in the magnilcation of the lens element L2. The lens element L2 is essentially a magnification changer and the lens element L3 is a focus corrector.

The following table sets forth the magnifications of the three lens elements L1, L2 and L3 for various magnifications of the entire system. In said table, the system magnification is designated MS, while M1, M2 and M3 designate the magnifications of the elements L1, L2 an`d L3, respectively.

lens element L1 can be determined for each angular position of barrel 16.

The foregoing is depicted in the graph of FIG. 3, wherein the abscissa represents the angular rotation of the barrel and the ordinate represents displacement along the optical axis. Curve C1 describes the distance X1 (see FIG. l) of the lens element L2 from the fixed lens element L1 throughout the angular rotation of the barrel and curve C2 describes the distance X2 (see FIG. 1) from the lens element L3 to the lens element L1 throughout f the same angular rotation. The axial distance X3 separating lens element L1 from specimen stage 10 is fixed at 114.905 mm. and does not change as the magnification of the system is varied.

A distance of 0.100 mm. separates lens L3B from lens L3A, a distance of 1.270 mm. separates lens L20 from lens L23, and a distance of 0.100 mm. separates lens L1B from lens L1A. The foregoing distances are taken along the optical axes and are fixed. The singlet and doublet of each of the movable eleme'nts L2, L3 move as a unit.

The following table more accurately describes the optical systems operation. Distances X1 and X4 in the table are given in inches. X1 again represents the distance from the fixed lens element L1 to the movable lens element L2 and X4 (see FIG. 1) represents the distance between the movable lens elements L2 and L3 for various angular positions 0 of barrel 16 or 16A. The table also sets forth the magnification MAG for the system for each of the illustrative 0 positions between the minimum and maximum limits of rotation, that is, between 0 and 339. The

tables distances are taken with an object at point P in proper focus and along the optical axes.

A pair of conventional erectors 14 are included in the optical system 8 and one of the erectors is disposed along each optical axis, above the lens element L3 of that axis. The erectors are shown only symbolically since they are well known. The erectors serve to erect the inverted images formed by the optical Syb-systems. In general, the optics of the duel eyepieces which can be used in conjunction with the objective system of my invention are well known.

The presentA invention is directed to the particular combination of lenses (a singlet and a doublet) in each of the lens elements along each axis. The problem encountered in variable magnification optical systems for stereomicroscopes is that it is quite difiicult to correct to a sufiicient degree the various aberrations introduced as a result of the continuously movable lenses. I have found that with the particular lens dimensions and characteristics set forth in the table of FIG. 2, a satisfactory degree of correction for the aberrations is achieved for the required specifications for a commercially acceptable instrument. In said table, the radii R refer to the various faces of the lenses as shown in FIG. 1 of the drawing and are given in millimeters. The thickness value T for each lens refers to the thickness of the central portion thereof taken along the optical axis of the lens and is also given in millimeters. The refractive indices ND and dispersion of the lenses are also set forth in the table. Using the lenses defined by the table of FIG. 2, a well corrected system -is achieved over the entire magnification range from .683 to 4.1.

In the course of the design of the specific embodiment disclosed in the drawing it was discovered that, in general, singlet-doublet pairs are the most suitable type of lens components for use in variable magnification stereomicroscopes of high quality of the type having specifications as disclosed herein.

The use of singlet-doublet pairs as components provides sufiicient variables in each component-to control optimally the first order color aberrations and the third order monochromatic aberrations (spherical aberration, coma, and tangential curvature of field) throughout the range of magnification and also permits the higher order aberrations to be held to acceptable limits. Components consisting of broken doublets (the two elements being closely adjacent but with their facing radii in general unequal) would permit the same control over first order color and third order monochromatic aberrations but for the desired field and numerical aperture the higher order aberrations would attain unacceptable levels. Another advantage of the use of such pairs, and particularly the specific pairs described above, is that the overall length of the objective section of the microscope may be relativelyl small since the maximum separation between lens elements L1 and L3, or the maximum value of the sum of X1 and X4, may be made less than that encountered 1n prior art stereomicroscopes.

I claim:

1. An optical system for the objective section of a stereomicroscope, the system comprising two optical subsystems, each sub-system having its object plane at essentially a fixed distance from its image plane, each subsystem having a stationary lens element nearest the object plane and a pair of movable lens elements along an optical axis, and means for moving the two movable lens elements along each optical axis at different rates to continuously vary the magnication of the objective system and to hold the image plane fixed, each of said stationary and two movable lens elements on each of said optical axes consisting of a singlet-cemented doublet pair, the optical characteristics ofthe lens elements along each of said optical axes being defined in the following table with thickness (T) and radii (R) being given in millimeters:

Lens Nd v T R R,=1oa.200 LM 1. 670 47.2 2.255 {112:53940 L1B 1.517 04.5 2.007 R,=33.e30 R4=54260 Llc 1.7500 27.8 1.900 R,=75.470 L 1.7505 27.8 1.727 R6=47.0s0 R1=24.s00 Lm 1.517 64.5 .082 11 g=10.400 @17.005 Lw 1. 670 47. 2 .000 {FSgAgos 1|=5 9. 1 L31 1. 07o 47. 2 1. 82s {Rmmao La 1.517 64.5 2.254 R=e0a0o R14=35.630 L30 1.7506 27.8 1.500 R,5=725.a00

2. An optical system for the objective section of a stereomicroscope in accordance lwith claim 1 wherein 6 the spacing X1, in inches, between the stationary lens element and the intermediate movable lens element along each axis, and the spacing X4, in inches, between the two movable lens elements along each axis are as follows:

Y {.1215 (when producing high magnification) 4 1.6613 (when producing low magnification) 3. An optical system for the objective section of a stereomicroscope in accordance with claim 2 wherein the mean distance between the stationary lens element and the nearest movable lens element is 0.80765 inch and the mean distance between the movable lens elements is 0.7699 inch.

1.7582 (when producing high magnification) .1229 (when producing low magnification) U.S. Cl. X.R. 

